A continuing program in laboratory animal nutrition involves various stocks and strains of rats, mice and guinea pigs as well as various species of nonhuman primates. A series of factorial designed feeding trials are used to evaluate the effect of diets made from different combinations of ingredients or those varying in nutrient concentrations on the growth, reproduction, maintenance and general health status of the species involved. The objective of this program is to improve the nutritional status of production and research animal colonies by providing diets with nutrient concentrations that are more consistent with the actual requirements. To accomplish this objective, open formula natural ingredient diets are developed. Data generated by this program are used as a basis for estimating the nutrient requirements of the species involved and also for ascertaining difference in nutrient requirements for stocks and strains within a species.